Knife sharpener



FIG. 1 is an enlarged perspective view of a knife sharpener, showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a left side view, the right side being a mirror image thereof;

FIG. 4 is a front view, the rear view being a mirror image thereof; and

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a knife sharpener, as shown and described. 